In the end I removed the project, did a clean checkout from git, and now it works again 🤷‍♂️

This issue is driving me nuts. And for me it happens literally every time I plug in my iPhone. Unplug, plug it back in, wait 5 minutes for Xcode to make the watch ready for development. Which then usually (but not always) fails by the way! Plug it back in, and you're just stuck waiting for 5 minutes yet again. When it fails, I get this error: Unable to prepare iPhone for development Could not locate the device support files. But building an app to my iPhone still works 🤷‍♂️